A shortage of human organs for transplantation has led to an interest in xenotransplantation using alternative non-human species such as the pig for transplantation. Studies of humans treated with living pig tissue to date have not found any evidence of PERV infection (Paradis et al 1999, Heneine et al 1998, Patience et al 1998, Pitkin and Mullon 1999, Schumaker et al 2000, Levy et al 2000, reviewed in Herring et a, 2001 and Cunningham et al 2001). These studies have included patients with a range of exposure routes to pig tissues, including skin grafts, transplantation with porcine pancreatic islets, and extracorporeal liver and splenic perfusion. Examination of primates transplanted with pig endothelial cells has also shown absence of infection with PERV, despite the analysis of multiple tissue types (Martin et al 1999), despite it being possible to infect non-human primate cells in vitro (Blusch et al 2000).
Despite the lack of evidence of PERV infection in humans, it would be desirable to use organ source pigs that gave the lowest possible zoonotic risk. Bosch et al (2000) found that most expressed PERV proviruses were defective, in common with endogenous retroviruses in other species such as humans, mice and cats (Tonjes et a/1999). Although some pig to pig variation in proviral distribution has been found (Jin et al 2000, Bosch et al 2000), some proviruses are likely to be common to all pigs. No systematic study has yet been published to determine which proviruses are intact, and what degree of pig to pig variance intact PERV proviruses have. Provirus expression patterns vary between different cell types (Langford et al 2001), or may be altered due to cryptic stimuli during organ rejection. Cloning of pigs by nuclear transfer is being actively explored as a method to generate genetically modified pigs for xenotransplantation (Polejaeva et al 2000).
Breeding pigs under specific or qualified pathogen-free conditions is generally assumed to reduce the risk of transmitting exogenous viral, bacterial, fungal and parasitic agents by xenotransplantation. However, microorganisms such as porcine endogenous retrovirus can be stably transmitted in the germ line and therefore cannot be easily eliminated. Many of the unique properties of retroviruses are due to the synthesis of a complement DNA copy from the RNA template via reverse transcriptase and integration of this DNA into the host genome. The integrated retroviral copy which is referred to as an endogenous copy or “provirus” can be transmitted via the germ line. Although many proviruses are defective and unable to replicate, if the provirus is intact, it can be activated by certain stimuli and then initiate viral replication using the host's cellular mechanisms. Replication of the virus may result in viremia, malignant transformation (e.g. via insertion of retroviral oncogenes), degeneration or other insertional effects (e.g. gene inactivation).
There is a clear need for a method of identification, isolation and screening for such retroviruses as porcine endogenous retrovirus. Further, such a method should be capable of being used in a rapid and efficient manner to screen herds of animals.